WDM (Wavelength Division Multiplexing) and ROADM (Reconfigurable Optical Add Drop Multiplexer) systems providing large capacity point-to-point connection have been introduced into core networks of optical communication. Following that, introduction of WDM and ROADM systems into metro and regional networks is also in progress.
In a node of a ROADM system, an optical path between a transponder disposed within the node or connected to the node and a route connected to the transponder is connected by any optional path and with any optional wavelength. That is, a ROADM node is provided with a function to connect an optical signal of a specific wavelength, among the wavelengths in a wavelength-multiplexed optical signal which is transmitted and received via a plurality of transmission lines, to a specific transponder.
FIG. 6 is a diagram showing one form of a ROADM network relevant to the present invention.
A ROADM node 601 is connected with other ROADM nodes 603 and 604 via respective ones of a plurality of optical fiber transmission lines 602. Then, the ROADM node 601 is controlled at each wavelength of an optical signal transmitting therein to perform transmission and reception of an optical signal to and from the other ROADM nodes 603 and 604.
Further, the ROADM node 601 combines optical signals transmitted by the optical transmitters of transponders 606 into a wavelength-multiplexed optical signal and sends out the optical signal to the optical fiber transmission lines 602. Receiving the wavelength-multiplexed light, the other ROADM nodes facing the ROADM node 601 wavelength-demultiplex the received light. Then, the optical receivers of transponders connected to respective ones of the other ROADM nodes receive the wavelength-demultiplexed optical signals.
FIG. 7 is a diagram for illustrating in more detail a configuration of the ROADM node 601 shown in FIG. 6. In FIG. 7, the ROADM node 601 comprises a wavelength path-line add/drop unit 723, transponder accommodation units 721 and 722, and a control unit 732.
The wavelength path-line add/drop unit 723 drops only an optical signal of a predetermined wavelength among wavelength-multiplexed optical signals received from transmission lines 701, and outputs the optical signal of the predetermined wavelength to the transponder accommodation unit 721. The wavelength path-line add/drop unit 723 and the transponder accommodation unit 721 are connected with each other by a plurality of routes 761.
Further, the wavelength path-line add/drop unit 723 adds to transmission lines 711 a wavelength-multiplexed optical signal which is output by the transponder accommodation unit 722. The wavelength path-line add/drop unit 723 and the transponder accommodation unit 722 are connected with each other by a plurality of routes 771.
The number of optical signal wavelengths included in the routes 761 and that in the routes 771 vary in accordance with the operating state of the ROADM node 601.
The transponder accommodation unit 721 connects the routes 761 and the transponders 751 such that a predetermined one among one or more transponders 751 receives an optical signal dropped from the transmission lines 701.
The transponder accommodation unit 722 connects the routes 771 and the transponders 751 such that an optical signal transmitted by the transponders 751 is added to a predetermined one among the transmission lines 711.
Here, the transmission lines 701 and 711 in FIG. 7 correspond to the optical fiber transmission lines 602 in FIG. 6.
The control unit 732 controls each unit of the ROADM node 601. Specifically, the control unit 732 controls the wavelength path-line add/drop unit 723 such that an optical signal to be a target of drop or add, among wavelength-multiplexed optical signals transmitted via the transmission lines 701 and 711, is connected to the transponder accommodation units 721 or 722.
Also, the control unit 732 controls the transponder accommodation unit 721 such that an optical signal dropped from the wavelength path-line add/drop unit 723 is received by a predetermined one among the transponders 751.
Further, the control unit 732 controls the transponder accommodation unit 722 such that optical signals transmitted by the transponders 751 are added, in the wavelength path-line add/drop unit 723, to respective predetermined ones of the transmission lines 711.
As an optical device provided with a function to connect any optional route with an optional transponder, which is required of the transponder accommodation units 721 and 722, there is mentioned a multicast switch described in Non-patent Document 1. Non-patent Document 1 describes a multicast switch which can connect eight transponders to four routes.
FIG. 8 is a diagram showing a configuration of a transponder accommodation unit employing a multicast switch, which is relevant to the present invention. In FIG. 8, for simple description, a case where four transponders (TPND) 804-1 to 804-4 are connected to a transponder accommodation unit 806 is illustrated.
The transponder accommodation unit 806 illustrated in FIG. 8 comprises a multicast switch 805 and variable wavelength filters 803-1 to 803-4. The multicast switch 805 comprises 1×4 splitters 801-1 to 801-4 and 4×1 switches 802-1 to 802-4. Here, the multicast switch 805 is different from the multicast switch described in Non-patent Document 1 in that the number of transponders possible to connect to it is four. However, basic operation of the multicast switch 805 illustrated in FIG. 8 is the same as that of the multicast switch described in Non-patent Document 1.
In FIG. 8, wavelength-multiplexed light is inputted from routes #1 to #4 to the transponder accommodation unit 806. The wavelength-multiplexed light inputted to the transponder accommodation unit 806 is split into four branches by the 1×4 splitters 801-1 to 801-4 comprised in the multicast switch 805. Then, the branches of the wavelength-multiplexed light are inputted to the 4×1 switches 802-1 to 802-4. The multicast switch 805 comprises the same number of 4×1 switches as the number of transponders connected to the multicast switch. In FIG. 8, since the number of transponders connected to the multicast switch 805 is four, the multicast switch 805 comprises four 4×1 switches.
The 4×1 switches 802-1 to 802-4 each select any one of optical paths inputted from respective ones of the 1×4 splitters 801-1 to 801-4 and connect the selected optical path with the corresponding one of the variable wavelength filters 803-1 to 803-4. As the result, wavelength-multiplexed light outputted from the 1×4 splitter connected to the selected optical path is outputted to the variable wavelength filter via the 4×1 switch.
The transmissive wavelengths of the variable wavelength filters 803-1 to 803-4 are each set to be coincident with the respective wavelengths of light to be received by the respective transponders which are connected to the variable wavelength filters. As a result of the operation of the variable wavelength filters 803-1 to 803-4, the transponders 804-1 to 804-4 each can extract and receive only light of a desired wavelength among a plurality of light components of different wavelengths included in the wavelength-multiplexed light.
In the configuration illustrated in FIG. 8, branching loss and transmission loss occur respectively in the 1×4 splitters 801-1 to 801-4 and in the 4×1 optical switches 802-1 to 802-4. For the purpose of compensating such losses, a configuration of disposing an optical amplifier on each route is considered.
FIG. 9 is a diagram showing a configuration where an optical amplifier is arranged on each route connected to the transponder accommodation unit. In FIG. 9, optical amplifiers for loss compensation 907-1 to 907-4 are arranged on the respective routes of the transponder accommodation unit 806 shown in FIG. 8. The optical amplifiers 907-1 to 907-4 are typically optical fiber amplifiers using an optical fiber doped with a rare-earth element as an amplification medium. The optical amplifiers 907-1 to 907-4 each includes an optical fiber being an amplification medium and an excitation light source inside, and controls their output levels or gains by controlling the output power of the respective excitation light sources.
It is assumed that the transponder accommodation unit 806 accommodates four wavelengths with respect to each route and deals with four routes. There, on all of the routes #1 to #4 connected to the transponder accommodation unit accommodating four transponders, optical amplifiers 907-1 to 907-4 are disposed respectively.
In the configuration shown in FIG. 9, the number of wavelengths amplified by one optical amplifier depends on status of transponders usage, that is, status of route and wavelength selections during operation, and accordingly takes a minimum value of zero and a maximum value equivalent to the number of accommodated transponders. For example, when the number of transponders is four, the maximum amplification performance required of one optical amplifier is that for amplifying four wavelengths, which corresponds to the case of using all of the transponders for one route. On the other hand, in practical operation, it is usually the case that only optical signals of the wavelengths to be received actually by the transponders are inputted from the routes #1 to #4 to the transponder accommodation unit 806 via the optical amplifiers 907-1 to 907-4. That is, there are some cases that four wavelengths are not amplified in each of the optical amplifiers 907-1 to 907-4 simultaneously.
Further Japanse Patent Application Laid-Open No. 2003-283019 relevant to the present invention describes a configuration of distributing excitation light by the use of a variable splitter and using the distributed light components respectively for different optical amplification means.